1. Field of the Invention
The present invention relates to a process for producing silanes by reducing polyhalosilane with use of a mixture of alkyl aluminum hydride and trialkyl aluminum as a reducing agent.
2. Description of the Prior Art
Silanes are industrially important compounds useful for various purposes such as the raw materials for organic and inorganic syntheses, fuel, catalyst etc.
Production of silanes by reducing polyhalosilane with alkyl aluminum hydride or the like is already known.
For example, the Japanese Patent Publication No. Sho 36-517, British Pat. No. 823,483, German Pats. Nos. 1,055,511 and 1,117,090 disclose the addition of a small amount of diethyl aluminum monochloride in the reduction of tetrachlorosilane with sodium hydride.
However, as will be apparent from the amount of use, said diethyl aluminum monochloride is simply used, through the formation of a certain complex, for activating sodium hydride by solubilizing the same which is practically insoluble in the solvent.
Also the French Pat. No. 1,499,032 discloses a process of reducing halosilane with highly pure alkyl aluminum hydride.
However, alkyl aluminum hydride, being industrially synthesized from metallic aluminum, hydrogen and trialkyl aluminum, is inevitably obtained as a mixture containing unreacted trialkyl aluminum, so that easily available inexpensive alkyl aluminum hydride is generally in the form of such mixture. Such mixture, if employed simply in the reduction of polyhalosilane, will not only result in a significantly lower yield of silanes than in the case of reduction with highly pure alkyl aluminum hydride but also lead to an increased formation of halosilanes, such as monochlorosilane, presumably from incomplete reduction of polyhalosilane and of ethane resulting from by-reactions.
For such reason, highly pure alkyl aluminum hydride has to be separated by distillation from the above-mentioned mixture, but in practice complete separation by distillation alone is extremely difficult because alkyl aluminum hydride and trialkyl aluminum not only have very close boiling points in certain systems but also form an azeotropic system.